where \(I_\lambda \)  is the specific intensity in a given direction and \(\mu =\cos \xi \) ,
with \(\xi \)  the angle between the given direction and the radial direction.
The intensity \(I_\lambda \)  is set by the radiation transfer equation, which reads [1]}
\(\hat{n}\cdot \nabla I_\lambda (\hat{n}) = \frac{dI_\lambda }{ds} = \alpha _\lambda \left(S_\lambda - I_\lambda \right),\) 
